It is highly desirable to be able to produce polyoxyalkylene glycols to a variety of molecular weights. The lower molecular weight materials (200 mwu to 1,000 mwu) are liquids which are used in many varied applications areas, like fiber lubrication. Higher molecular weight materials (2,000 mwu to 6,000 mwu) are useful in the preparation of soil release polymers by the reaction with terephthalic acid and its esters. Ultra high molecular weight materials (over 10,000 mwu) are useful as waxes in many personal care applications.
All of these applications require that the polyoxyalkylene compound used (a) be capable of being produced on a repeated basis to a specified average molecular weight (i.e. have a reproducible molecular weight distribution) and (b) to be able to stand up to thermal and other degradations in the various applications in which they are used.
All polyoxyalkylene derivatives have a common feature, that is the presence of an ether bond derived from the reaction of ethylene or propylene oxide; EQU --CH.sub.2 CH.sub.2 --O--CH.sub.2 CH.sub.2 --O-- or --CH.sub.2 (CH.sub.3)CH--O--
We have found that these bonds in polymeric species are degraded not only during the polymerization reaction but also after the reaction is complete. The degradation is accelerated by the presence of oxygen. Polyoxyalkylene chains break down into lower molecular weight species, and much of the performance attributes are lost.
This degradation is a major concern for the ultra high molecular weight species. While it is known in the art, that in order to achieve high molecular weight species of polyoxyalkylene compounds, exotic catalysts have to be used. But it was not until this invention, that it was recognized that the degradation could be arrested by incorporation of stable free radical compounds, most importantly nitroxides.
Antioxidants are well known, but the "standard antioxidants" are ineffective in the prevention of this specific degradation. The use of stable free radical compounds has surprisingly been found to be effective in preventing degradation. A variety of "standard antioxidants" have been shown to inhibit or retard vinyl polymerization. The commonly used inhibitors appear to function by reacting in some manner with an initiator radical to yield a species of lower reactivity that results in a lower tendency to continue chain propagation. These "standard antioxidants", which are ineffective in our invention, include phenols, quinones, aromatic nitro and nitroso compounds, amines and thiol compounds.
Stable free radicals have been known for many years and exist in the patent literature, but have been used primarily, if not exclusively in the prevention of vinyl reactive systems. U.S. Pat. No. 4,670,131 issued to Ferrell discloses the use of stable free radical compounds in the prevention of polymerization of olefinic materials. This process relates to the reaction of a vinyl containing material to make a saturated higher molecular weight species. Clearly, this reaction has been understood in terms of free radical chemistry. Free radical inhibitors or scavengers inhibit or retard the polymerization of chain propagating reactions in vinyl monomer systems. The mechanism is thought to be a scavenger of the free radicals which form on the monomer. Since the free radical scavenger is stable, it reacts rapidly with free radicals of the monomer, which form in low concentrations.
In addition to abstraction, several compounds retard vinyl polymerization by radical addition which again produces a radical species which is not reactive toward the monomer. Quinone type inhibitors are probably the best example of this kind of inhibition mechanism.
It was not originally appreciated that the degradation process of polyoxyalkylene compounds was free radical like in nature, since the typical inhibitors like phenols, quinones, aromatic nitro and nitroso compounds, amines and thiol compounds were of no value in stabilizing the polyoxyalkylene products. We have discovered that in order to develop an inhibitor for polyoxyalkylene polymers, a different type of approach must be employed. Stable free radicals, such as NOVA INHIBITOR 469, (a nitroxide type stable free radical) provide such a system not only for the polyoxyalkylene compounds but also for derivatives like soil release polymers. These stable free radicals are far too stable to initiate polymerization, but their free electron is available to immediately react with any radical initiator, rendering the potential initiator totally inactive. This class of inhibitors has been found to effective in protecting polyoxyalkylene polymers from free radical type degradation which reduces molecular weight.
The desired reaction forming polyoxyalkylene polymers is represented generically as follows ##STR1##
During the reaction, most importantly in molecular weights in excess of 1000 MWU, the terminal hydroxyl group can eliminate producing terminal unsaturation. The terminal unsaturation has the following structure; EQU HO--(CH.sub.2 CH.sub.2 O)(n-1)--CH.dbd.CH.sub.2
The continued polymerization of this compound to higher molecular weights is blocked by the conversion. We have surprisingly found that the inclusion of between 0.00000001 and 1.0 percent of a stable free radical into the diol prior to reaction will prevent the formation of the terminal unsaturated material allowing for the preparation of higher molecular weight more chemically pure polymers.
Additionally, the polyoxyalkylene chain can degrade into lower molecular weight products. This is observed particularly when aggressive conditions of temperature and catalyst concentrations are used in the polymerization reaction. ##STR2##
Inclusion of an effective inhibitory amount of stable free radical inhibits this reaction as well. The preferred stable free radical is a nitroxide.